光子晶体电光调制和粗波分复用集成器件研究 
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Objective With the advent of 5G communication, the demands for communication systems with increased capacity and speed are growing rapidly. Unit-device technology has basically matured, and the only way to break through the bottleneck of information systems to achieve high-speed, low power consumption, and small size is to use silicon-based optoelectronic integration. Various structures have been proposed for electro-optical modulators and wavelength-division multiplexers, and research on these two types of individual devices has become increasingly mature. However, there have been few studies which integrate the two devices in a single device to achieve multiple functions. Because silicon-based optoelectronic integration has the characteristics of low packaging cost, small size, and high integration, we propose here an on-chip integrated device for electro-optic modulation and coarse wavelength-division multiplexing. Integrating these two functions on the same silicon-based photonic-crystal plate yields a device with the desirable characteristics of small size, low insertion loss, small modulation voltage, large modulation depth, and low channel crosstalk. We therefore anticipate that this integrated-device study will be helpful for the rapid development of information technology in the near future.
Methods The integrated device described in this paper consists of two modules, a photonic-crystal electro-optic modulator and a coarse wavelength-division multiplexer. The integrated device is a photonic crystal with a heterogeneous structure, which achieves “on” and “off” state modulation of different wavelengths in the electro-optical modulation module, and the download function for the different wavelengths is provided by the coarse wavelength-division multiplexing module. This article is based on a simulation analysis using the FDTD and DEVICE modules in the commercial optical-simulation software Lumerical. First, an L3 resonant cavity and PN junction employ plasma dispersion to modulate the wavelength of the input signal. Then, using direct coupling between the microcavity and waveguide, the device utilizes the L3 resonant-cavity structure to obtain coarse wavelength-division multiplexing, and complete the design of the coarse wavelength division multiplexing module. Finally, the two modules are cascaded together, and the modulated input port and the wavelength-division-multiplexing output port utilize tapered waveguides to reduce the impact of mode mismatch between the photonic-crystal plate and the nanowire waveguide. Cascading the two modules shifts the resonant wavelength of the L3 cavity, so it is fine-tuned to complete the electro-optic modulation and coarse wavelength-division multiplexing at 1530 nm and 1550 nm design wavelengths.
Results and Discussions The integrated device has excellent performance, and the modulation voltage is small. When the modulation voltage is 1.505 V, the change in electron concentration reaches ΔNe=6.31×10 18cm3, and the change in hole concentration reaches ΔNh=7.94×10 18cm3 (Fig. 6), which modulates the wavelength (Fig. 7). According to the simulation, the transmittances of the integrated device at 1530 nm and 1550 nm in the “on” state are respectively 85.10% and 80.38%, while the corresponding transmittances in the “off” state are 0.68% and 0.5%, respectively. We calculate that the total insertion loss is less than 0.95 dB, the extinction ratio is greater than 20 dB, the modulation depths D are both 0.99, and the channel crosstalk values are all less than -27.59 dB (Table 1), which indicates good device performance for both modulation and wavelength division.
Conclusions This paper presents an on-chip integrated device for electro-optical modulation and coarse wavelength-division multiplexing. The L3 resonant cavity of the photonic crystal and the silicon-based photonic-crystal waveguide are coupled together, and the modulation and wavelength-division multiplexing functions are obtained by adjusting the width of the waveguide and the resonant cavity. The tapered structure of the input/output waveguide reduces the coupling loss and increases the transmittance. The simulation results show that the integrated device can complete the modulation and multiplexing of two wavelengths. Compared with previous research results, this is no longer a single-function device. The insertion loss of the device at the working wavelengths of 1530 nm and 1550 nm is 0.70 dB and 0.95 dB, respectively, the extinction ratios are 20.97 dB and 22.05 dB, the modulation depths are both 0.99, the channel crosstalk is -29.05 dB and -27.59 dB, and the device size is only 17.83 μm×17.3 μm×0.22 μm. The compact structure of the modulator makes it easy to integrate into applications for high-speed optical-communication systems. We therefore believe that this device has huge application potential for the development of integrated optical devices and high-speed large-capacity communication systems.
1 引言
随着5G通信的到来,人们对通信系统的容量和速度的需求日益增加,而目前单元器件技术已基本成熟,突破信息系统走向“高速率”、“低功耗”、“小尺寸”等瓶颈的必由之路是硅基光电子集成[1]。实现多种功能器件的系统集成已逐渐成为目前所关注的热点问题。硅基光电子集成是通过互补金属氧化物半导体(CMOS)工艺,在基于硅材料的衬底上,将光子和光电子功能器件集成在同一个芯片上。硅基光电子具有封装成本低、尺寸小、集成度高等特性,对未来信息技术的高速发展具有重大的研究意义。
电光调制器是光通信中的一个重要器件。近年来,不少学者提出了不同结构的电光调制器,如脊波导型调制器[2-3]、马赫-曾德尔型调制器[4-7](MZM)、基于狭缝波导的聚合物型调制器[8]和光子晶体谐振腔(PCR)型调制器[9]等。2017年Daud等[10]研究设计出了高品质因数(Q~105)光子晶体谐振腔型电光调制器,以微小的折射率差就可实现高达GHz量级的快速调制。但是脊波导型调制器和后三种调制器相比,尺寸较大,不利于集成。与基于狭缝波导的聚合物型调制器相比,纯硅的PCR型调制器可以与标准的半导体工艺完美结合,且制作成本更低[11],因此,基于纯硅的PCR型调制器更适用于集成。
同样地,波分复用器也在波分复用光通信中具有重要的地位。波分复用可以分为粗波分复用与密集波分复用,两者之间最大的区别是波长间隔。粗波分复用的信道间隔是20 nm,而密集波分复用的信道间隔一般是0.8 nm或者0.4 nm。目前也设计出许多不同结构的波分复用器,如阵列波导光栅(AWG)型[12]、微环谐振腔(MRR)型[13-14]和PCR[15-18]等。2018年Yang等[19]提出了一种基于硅片集成八通道波分复用/解复用器的设计方案,该解复用器由高Q值在105以上、自由光谱范围在200 nm以上平行排列的一维光子晶体纳米束腔组成。器件总尺寸大小仅为12 μm×15 μm。从器件尺寸和自由光谱范围这两个方面考虑,PCR型的波分复用器件因其尺寸小、自由光谱范围大,在硅基光电子器件集成领域更受欢迎。
综上所述,目前研究人员已提出多种结构的电光调制器和波分复用器,对这两类单一器件的研究日趋成熟,但对两者进行集成实现器件多种功能的研究却很少。本文提出一种基于光子晶体的电光调制和粗波分复用集成器件。该集成器件在工作波长1530 nm和1550 nm下都可以完成电光调制和双通道波长的复用,具有插入损耗较低、消光比较高、调制深度大和信道串扰小的特点,性能优良,这对未来光子集成网络研究具有重要意义。
2 理论模型与原理分析
本文所提出的电光调制和粗波分复用集成器件的理论模型如
图 1. 基于光子晶体的电光调制和粗波分复用的集成器件的理论模型
Fig. 1. Theoretical model of integrated device based on photonic crystal electro-optic modulation and coarse wavelength division multiplexing
电光调制模块的工作原理是等离子体色散效应,当工作波长在1550 nm附近时,硅基材料的吸收系数和折射率随着载流子浓度变化的对应关系式为[20]
式中:Δαc为单位长度吸收系数的变化;Δnc为材料中折射率的变化;ΔNh、ΔNe分别为空穴和电子浓度变化(单位:cm-3)。由(1)式和(2)式可知,随着载流子浓度的增大,吸收系数逐渐增大,而折射率将逐渐减小。根据(2)式,可得到调制模块L3谐振腔中的载流子浓度变化量与折射率变化量的关系。
粗波分复用模块则是利用L3腔与波导直接耦合形成的滤波器,其理论模型如
式中ω0为谐振腔的谐振频率。
图 2. L3型谐振腔-波导滤波器理论模型。 (a)简化之前的模型;(b)简化之后的模型
Fig. 2. Theoretical model of L3 resonator-waveguide filter. (a) Model before simplifying; (b) simplified model
当光仅从S+1入射至S+2=0时,联立(3)~(5)式可得
经过上述分析可知,当入射光的谐振频率与L3腔的谐振频率一致时,通过调节窄波导的宽度就可实现良好的滤波功能。
3 结构设计与参数优化
基于光子晶体的电光调制和粗波分复用的集成器件的结构如
图 3. 光子晶体的电光调制和粗波分复用的集成器件的结构
Fig. 3. Structure of integrated device of photonic crystal electro-optic modulation and coarse wavelength division multiplexing
3.1 电光调制器模块
图 4. 电光调制器模块的结构图和参数变化曲线图。(a)电光调制器结构示意图;(b)L3腔的谐振波长与反射率随谐振腔宽度W1的变化图
Fig. 4. Structure diagram and parameter change curves of electro-optic modulator module. (a)Schematic diagram of electro-optic modulator structure; (b) variation of the resonant wavelength and reflectivity of the L3 resonator with the resonator width W1
调制器模块的电学掺杂结构如
图 6. 两端电压为1.505 V时载流子浓度分布图。(a)P型载流子浓度;(b)N型载流子浓度
Fig. 6. Carrier density distributions diagram when the voltage across both ends is 1.505 V. (a) P-type carrier density; (b) N-type carrier density
图 7. 调制分别在V=1.505 V“通”和V=0“断”状态下的反射谱图
Fig. 7. Modulation reflectance spectra under V=1.505 V “on” and V=0 “off” states
3.2 粗波分复用器模块
粗波分复用器模块的本质是由多信道的滤波器组成。该模块采用两个晶格常数不同的异质结构光子晶体,分别实现波长为1530 nm和1550 nm的滤波功能,如
图 8. 粗波分复用模块结构图
Fig. 8. Structure diagram of coarse wavelength division multiplexing module
图 9. 粗波分复用模块的透射谱图
Fig. 9. Transmission spectrum of coarse wavelength division multiplexing module
3.3 电光调制器与粗波分复用器级联模块
将1530 nm的电光调制器与其对应的滤波器级联,如
图 10. 波长为1530 nm的电光调制器和滤波器级联设计图。(a)级联器件的结构参数示意图;(b)锥形波导结构参数示意图;(c)输入波导优化前后透射谱图
Fig. 10. Design of cascaded electro-optic modulator and filter with a wavelength of 1530 nm. (a) Schematic diagram of structural parameters of cascaded devices; (b) schematic diagram of tapered waveguide structure parameters; (c) input transmission spectrum before and after optimization of the waveguide
4 性能分析
电光调制器的性能指标有调制电压、插入损耗、消光比、调制深度、调制速率和能耗等。调制深度越大,表示调制器越有利于长距离、低误码率传输,调制效果越好。调制速率是指调制设备单位时间可调制的码元速率,在电光调制器中能反映“通”、“断”调制的转换速度。能耗是指传输1 bit的信息所需要的平均能量。粗波分复用器的性能指标有插入损耗和信道串扰等。
本研究采用Lumerical仿真软件中的时域有限差分法(3D-FDTD)分析了集成器件的调制电压、插入损耗、消光比、调制深度和信道串扰值。仿真计算时,如
图 12. 集成器件的稳态场分布图。(a)1530 nm的调制状态为“通”;(b)1530 nm的调制状态为“断”;(c)1550 nm的调制状态为“通”;(d)1550 nm的调制状态为“断”
Fig. 12. Steady-state field distribution diagrams of integrated devices. (a) Modulation state of 1530 nm is “on”; (b) modulation state of 1530 nm is “off”; (c) modulation state of 1550 nm is “on”; (d) modulation state of 1550 nm is “off”
表 1. 集成器件在不同波长下的性能参数
Table 1. Performance parameters of integrated devices at different wavelengths
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图 13. 集成器件的透射谱图。(a)调制状态为“通”;(b)调制状态为“断”
Fig. 13. Transmission spectra of integrated device. (a) Modulation state is “on”; (b) modulation state is “off”
该集成器件在波长分别为1530 nm和1550 nm时的总插入损耗γIL、消光比ηER、调制深度D和信道串扰值CCT如
5 结论
提出一种用于电光调制和粗波分复用的片上集成器件。采用光子晶体L3型谐振腔和硅基光子晶体波导之间的耦合,通过调节波导宽度和谐振腔宽度,可以实现调制和波分复用功能。采用锥形结构的输入/输出波导可以减小耦合损耗,提高透射率,仿真结果表明该集成器件可以完成两个波长的调制及复用。与之前的研究成果相比,该集成器件不再是功能单一的器件。此外,该结构紧凑,易于集成,可应用于高速光通信系统,在集成光学器件的发展及高速大容量通信系统中有巨大的应用潜力。
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